Protection of subsea wellheads is a technology that is in its infancy. The use of large diameter subsea caissons have been tried in the past for Artic areas, but have been generally unsuccessful mainly due to installation problems. The practice of dredging "glory holes" is currently used but is expensive and does not protect equipment from scour debris. Piled frameworks have been used for protection against fishing gear and anchors but again they are large, difficult to install and expensive. Recently the industry has been developing insert tree systems, with the intent of reducing the quipment size so that it can be housed inside a conductor pipe. Although this approach meets many primary requirements it also introduces reliability severe and maintenance problems.
Conventionally, therefore, subsea wells have the wellhead at the sea bed and have connected to them either a blowout preventer during the pulling phase or a valve block assembly and flow line connector during the production phase. The wellhead and the associated equipment project somewhat above the sea bed and are thus vulnerable to damage.
All equipment on the ocean floor is at risk due to a variety of hazards. In offshore hydrocarbon developments to date, the major hazards to subsea equipment are from trawlboards used in commerical fishing and from anchors used by supply boats, laybarges, drill ships, etc. For hydrocarbon development in cold water regions, a new hazard which must be considered is ice scour of the sea bottom. For example in the Beaufort Sea area, ice scouring occurs in the region beyond landfast ice where ice ridges are formed and frequently scour the seabed. Off the east coast of Canada very deep scours can occur when an iceberg grounds out. Therefore there is a requirement for a well completion design that will maintain control over the well even in the extreme event of an ice scour shearing off the top of the well.
One conventional solution consists of placing pressure control devices (such as master valves, DHSV's, etc.) and the wellheads with their sealing systems below the scour zone together with suitable shear devices to insure their integrity. There are many alternate ways to implement such a solution. One method is to excavate a large glory hole with sufficient depth to place all well control devices below the scour line. The glory hole approach is a good technical solution but is not economically attractive. The other end of the spectrum of solutions, is to design compact Xmas trees that can be installed inside the well bore to the required depth with sacrificial equipment above the well control devices. The insert tree has been developed to minimize equipment heights above the mudline but it is unknown whether the equipment between the surface and the well control devices is sacrificial.
However, between the two extremes described above, there are a number of other solutions available. They basically consist of installing a caisson large enough to allow a conventional subsea tree to be installed inside the caisson. The success of this approach depends on the feasibility of installing the caissons.
The insert tree evolved from a need to protect subsea wellheads from snagged anchors and trawlboard impacts. One protection method that has been tried is to install protective covers that divert, deflect, or snag fishing gear and anchors. These structures are large and expensive to install beacuse of the height of trees above mudline. Therefore, compact trees were developed to reduce the wellhead profile. This leads to the development of the insert tree which places the complete tree assembly inside the wellbore with only the flowline connection protruding above the mudline.
One company has developed a tree that will fit into a 30 inch casing. It uses ball valves and inline operators to keep the valve assembly inside the 30 inch diameter. Another company offers an insert tree that uses gate valves and in-line operators but requires a 40 inch casing.
The main advantage of the insert tree is that the basic drilling procedures are not affected. The structural casing run into the hole will require a casing hanger at the required elevation to receive the wellhead. Special running tools and a temporary wellhead extension will also be required. Except for these special tools and equipment, standard drilling practices can be used.
The main disadvantage of the insert tree is some loss in operational flexibility: no in-situ repair or inspection of master valve block and operators is possible. The equipment must be retrieved for all levels of repair. Direct surface well access is not possible except by first removing the flowline extensions. TFL systems must be relied upon as a primary method of well bore access. Wing valves have not been incorporated in the current design of insert trees. Therefore, normal shut-in practice will be to stop the flow with a surface valve to avoid excessive wear on the master block valve. Placement of compact valve assemblies in restricted confines could complicate installation, retrieval, and well killing operations. Blowout preventers are still exposed during the drilling phase and, because the production tubing exits through the top of the casing, the wellhead assembly still protrudes significantly above the mudline, probably six or seven feet.
To overcome some of these problems, it has been proposd to make a caisson large enough to contain the blowout preventer. U.S. Pat. Nos. 3,344,612 and 3,796,273 describe methods of installing such a caisson type. U.S. Pat. No. 3,344,612 uses a jetting technique for caissons installed in a soft sea bed and U.S. Pat. No. 3,796,273 uses a rotary drilling technique for hard sea bed. In this latter arrangement, the base of the caisson has cutting teeth on its surface and the complete caisson is rotated thereby boring its own hole. Although these techniques appear feasible, they require a drilling rig which is very expensive and must rely on cementing to ensure their soundness since the surrounding soil is highly disturbed. In the case of the rotating caisson, the feasibility of easily rotating such a large body when it nears its full penetration is questionable since there is a large surface area at a large radius.
U.S. Pat. No. 3,380,256 proposes putting a complete drill rig in a caisson and sinking it by means of hydrostatic pressure. In this patent, the caisson skirt is pushed into the sea bed until the base of the caisson contacts the sea bed with the result that part of the caisson with the wellhead equipment still protrudes above the sea bed. The technique of using hydrostatic pressure described in this patent has been used successfully in sinking suction anchors. The main limitation is the depth of penetration obtainable before the soil-resisting force balances the force produced by the hydrostatic pressure.